Treatment of waste products with anaerobic digestion

ABSTRACT

A waste stream is treated by anaerobic digestion. A process is described involves a step of separating solids from digestate, and returning separated solids to a digester. Optionally, there may be a step of solids separation in which larger solids are removed from the digester. A process and apparatus are described for treating waste sludge from a wastewater treatment plant in an anaerobic digester. Feed sludge is thickened or solids are separated from digestate and returned to the digester. Additional co-digestion waste may be added to the digester. The process and apparatus may be used in a retrofit of an existing wastewater treatment plant.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Application Ser. No.62/108,145, filed Jan. 27, 2015, and U.S. Application Ser. No.62/265,691, filed Dec. 10, 2015. U.S. Application Ser. Nos. 62/108,145and 62/265,691 are incorporated by reference.

FIELD

This specification relates to a process or apparatus for treating wasteproducts, such as waste sludge from wastewater treatment or agriculturalor industrial wastes, involving anaerobic digestion.

BACKGROUND

A biogas may be produced through the anaerobic digestion of a materialcontaining biomass. The biogas is typically comprised of 50-75% methaneand 25-50% carbon dioxide. Other gases, such as nitrogen, hydrogen,hydrogen sulfide or oxygen may be also present but collectively areunlikely to account for more than 10% of the biogas. Of these othergases, nitrogen is likely to be the largest component. The biogas can beburned directly with oxygen, for example, and so is usable as a fuel.The methane within the biogas can also be concentrated to provide areplacement for natural gas.

Biogas can be produced in an anaerobic digester. The digestion processinvolves microorganisms, primarily bacteria, which break down or convertthe input materials to produce the biogas and an effluent. The processinvolves a series of bacteria types and processes, primarily hydrolysis,acidogenesis, acetogenesis and methanogenesis.

Anaerobic digesters were originally designed primarily for use withcattle manure and sludges. The sludge may be, for example, waste sludgefrom a municipal wastewater treatment plant. Municipal wastewater, orsewage, is typically treated using an activated sludge process withprimary clarification, a biological process train, and secondaryclarification. Waste activated sludge, optionally thickened, and primarysludge may be fed to an anaerobic digester at a dry solids (DS)concentration of up to about 4%. The digester typically operates at 2 to2.5% solids concentration in the digester with a 20 to 25 day hydraulicretention time (HRT).

SUMMARY

This section is intended to introduce the reader to the more detaileddisclosure that follows, and not to limit or define any claimed ordisclosed invention. One or more inventions may reside in a combinationor sub-combination of one or more apparatus elements or process stepsdescribed in this document.

Processes and apparatus will be described in this specification fortreating a waste involving anaerobic digestion. The waste may be, forexample, an agricultural or industrial waste, or a waste sludge from awastewater treatment plant. The process involves a step of separatingsolids from digestate, and returning separated solids to a digester.Optionally, there may be a step of solids separation in which largersolids are removed from the digester.

A process and apparatus will be described in this specification fortreating waste sludge from a wastewater treatment plant. The wastesludge is treated in an anaerobic digester. Feed sludge is thickened orsolids are separated from digestate and returned to the digester.Additional co-digestion waste may be added to the digester. The processand apparatus may be used in a retrofit of an existing wastewatertreatment plant.

DRAWINGS

FIG. 1 is a schematic flow sheet of an anaerobic digestion system.

FIG. 2 is a schematic flow sheet of an anaerobic digestion system foruse in combination with a wastewater treatment plant with sludgepre-thickening.

FIG. 3 is a schematic flow sheet of an anaerobic digestion system foruse in combination with a wastewater treatment plant with recuperativesludge thickening.

FIG. 4 is a graph showing the effect of polymer dose on dewateringdigestate from digesters with and without recuperative thickening.

DETAILED DESCRIPTION

FIG. 1 shows a plant 10 for treating a feed liquid 12 containing organicmatter. The feed liquid 12 may be a single stream or a composite,whether mixed or not, of two or more waste streams. The feed liquid 12may have a low solids content, for example a total solids (TS)concentration of 6% or less or 4% or less, measured on a dried solids(DS) basis. The feed liquid 12 may be pre-separated to removecontaminants such as plastic, glass, metals or other un-digestiblesolids. As examples of feed liquids, wastewater from a slaughter houseor food processing plant may have a TS concentration of 1-2%. Pig manuremay have a TS concentration of 2-3%. Waste sludge from a wastewatertreatment plant using an activated sludge process may have a TSconcentration of 1-4%.

The feed liquid 12 optionally flows into an upstream thickener 14. Theupstream thickener 14 is a solid-liquid separation device such as aclarifier, sedimentation basin, flotation device, press or filter, or acombination of one or more of these or other devices. The firstthickener 14 produces a first thickener effluent 16. The first thickenereffluent 16 is primarily water, with a TS concentration typically of 2%or less, that may be treated for example as municipal waste water. Theupstream thickener also produces, as a retained portion, a thickenedfeed liquid 18. The thickened feed liquid may have a TS concentrationof, for example 6 to 12% or more.

The feed liquid 12 or thickened feed liquid 18 flows into a digester 20.The digester typically 20 comprises one or more tanks, in series or inparallel or both, with mixing apparatus. For example, the digester 20may be a sealed tank with an internal mechanical mixer. A suitabledigester 20 and mixers are available from UTS Products GmbH. Thedigester 20 contains microorganisms, primarily bacteria, to digest thefeed liquid 12 or thickened feed liquid 18. The digester 20 may beseeded with the microorganisms, or the microorganisms may be carriedinto the digester 20 as a component of the feed liquid 12 or thethickened feed liquid 18. The microorganisms convert solids in the feedliquid 12 or thickened feed liquid 18 into, among other things, a biogas22 which is collected and removed from the digester 20.

The digester 20 also produces a digestate 24. Due to the action of themicroorganisms in the digester 20, the digestate 24 has a reduced TSconcentration relative to the feed liquid 12 or thickened feed liquid18, whichever is fed to the digester 20. The digestate 24 may have a TSconcentration of for example 3-8%, typically 5-6%.

Optionally, the digestate 24 may pass through a separator 26. Theseparator 26 provides a coarse separation, for example as produced by astatic screen, vibrating screen, screw press or similar equipment. Aportion of the separated solids 28 from the separator 26 may be returnedto the digester 20, but the separated solids 28 are preferably removedfrom the plant 10. The separated solids 28 are likely to have highconcentrations of materials that are difficult for anaerobic bacteria todigest. The separated solids 28 may be processed further to produceuseful products such as fertilizer. Although the separator 26 is shownin FIG. 1 in an effluent stream, the separator 26 may also be located ina re-circulating side stream, alone or in combination with one or moreother unit process such as thickeners or other solid-liquid separationdevices, a heater, or a grinder.

The separator 26 may be used without other solid-liquid separationdevices operating on the feed liquid 12 or digestate 24. Preferably, theseparator 26 is used in a process of selective solids recovery incombination with one or more other solid-liquid separation processesoperating on the digestate 24 such as a second thickener 34 to bedescribed below. The other solid-liquid separation device may be locateddownstream of the separator 26, or in a re-circulating side stream. Theother solid-liquid separation device is used to retain certain solidsincluding biomass to increase the solids retention time (SRT) of biomassin the digester 20, and biomass inventory, without increasing thehydraulic retention time (HRT) of the digester 20. The separator 26, incontrast, is used to remove less desirable solids to decrease their SRT.

When digesting some forms of waste, the feed liquid 12 may have largesolid particles. This tends to be the case with high solids contentfeeds, and also with feeds that start with large particles but cannot bescreened prior to feeding the digester 20. This is the case, forexample, with fruit and vegetable processing peel waste, animal manuresthat contain bedding materials, silage, and other fibrous substrates.These substrates are usually fed with chopper pumps or conveyors,depending on the solids content, and usually undergo some level ofgrinding prior to entering the digester 20. However, the particle sizeusually remains large, for example 4 mm or larger. When digesting thistype of substrate, a portion of the large particles are not digestedeven though they may be volatile in principle.

There is no advantage in retaining large particles of undigested solidsin the digester 20, as there is a limit to the extent to which theseparticles can be anaerobically degraded, even if they contain volatilematter, in any reasonable SRT. Undigested solids tend to accumulate in adigester 20 and do not contribute significantly to biogas generation. Anaccumulation of undigested solids thickens the digestate 24, makingmixing in the digester 20 more difficult and causing an increase in themixing energy consumed. Solids recovery preferably extends the SRT ofthe biomass, but not of the undigested solids. By using two or moreseparate solid-liquid separation processes, two or more separate SRTscan be maintained, one for each type of solid preferentially retained orremoved by the solid-liquid separation processes.

Undigested solids tend to be larger than useful biomass and this sizedifference may be used to remove undigested solids while retainingbiomass. The separator 26 may be used to remove undigested solidswithout removing significant amounts of biomass. The separator 26 mayuse, for example, a filter screw press with an appropriate screenopening size for the particular substrate or combination of substratesbeing fed to the reactor 20. Suitable filter presses are manufactured byUTS Products GmbH and others. No polymer is required in the press sinceit is not intended to remove biomass or suspended solids that may stillbe digestible. Pressure generated by a scroll pushing solids against aspring or pneumatically loaded pressure plate forces liquid andsuspended and colloidal solids through the screen, while largerundigested solids exit as a 20 to 30% solids cake 28. The cake 28 may besent for composting or disposal, or for use as a bedding material orother uses.

The screen opening in the press, or the equivalent opening in anotherdevice, may be 300 microns or larger. The microbial biomass solids aremuch smaller in size, usually smaller than 5 microns, and they arepushed out with the liquid through the screen by the pressure developedinside the press. The separator effluent 30 flowing out of the filterscrew press may contain from 1 to 4% total suspended solids (TSS),comprising bacteria, smaller undigested solids and possibly inert fines.

The digestate 24 may pass through a grinder, for example in a sidestream loop or upstream of the separator 26, to grind large volatilesolids particles into a practically digestible size. This can reduce theamount of solids that are considered undigestible and removed in theseparator 26.

The separator effluent 30 may go to a second stage of solids separation,as will be described below, or may be returned to the digester 20.Preferably, a portion 32 of the separator effluent 30 is also wasted toa dewatering device to avoid an accumulation of small inert particles,such as silt, and of biomass. Additionally or alternatively, a portionof a thickened solids stream, such as sludge 38 (to be described below)from the second thickener 34, can be wasted to a dewatering device; ordigestate 24 can be wasted directly from the digester 20 to a dewateringdevice. Solids from the de-watering device can be sent, for example, forcomposting or land application as allowed. De-watering liquid can betreated as municipal wastewater.

If the digester 20 is co-located with a wastewater treatment plant, thenany liquids to be treated as municipal wastewater may be sent to thehead of the wastewater treatment plant.

Separator effluent 30, or digestate 24 if there is no separator 28,flows to the second thickener 34. The second thickener 34 has a smallerseparation size than the separator 26. For example, the second thickener34 may be a filter with a screen or mesh having an opening size in therange of about 10-200 microns or smaller. The second thickener 34 may bea drum filter, disc filter or similar equipment. The second thickener 34produces a sludge 38 with an elevated solids content, for example a TSconcentration of up to 10% or more. The TS concentration of the sludge38 is more than the TS concentration of the digestate 24. A significantportion, or example 50% or more up to 100%, of the sludge 38 ispreferably recycled to the digester 20. The recycled sludge 38 increasesthe SRT of the digester and its operating TS concentration, which is theTS concentration of the digestate 124 when the digester is acontinuously stirred tank reactor (CSTR).

The second thickener 34 may be used without an upstream separator 26when the suspended solids in the feed 12 tends to have a small particlesize, usually under 3 mm, which are degradable in varying degrees. Thisis often the case for digesters 20 that are fed with waste sludge from amunicipal waste water treatment plant, typically with a suspended solidscontent of 2 to 4% total suspended solids (TSS), or industrial effluentswith high soluble chemical oxygen demand (COD) content and lower TSScontent.

As soluble and particulate volatile solids are digested, microbialbiomass is created. This biomass includes anaerobic bacteria that growslowly and so have low biomass yields. A biomass retention time of 25 to30 days or more is preferred for stable and reliable operation of thedigester 120. However, if the liquid fed with the solids was to remainin the digester 20 for 30 days, giving a hydraulic retention time (HRT)of 30 days, in many cases the tank volume could be excessive and noteconomical to construct and mix. Therefore SRT for solids in the sizerange including the biomass is increased relative to the HRT byretaining solids with the second thickener 34 and returning at least aportion of them to the digester 20 in sludge 38. Undigested yet readilydigestible solids may be similarly retained and returned to the digester20.

The second thickener 34 is preferably closed to inhibit ammonia andhydrogen sulfide gases from escaping to control odors. Various solidsseparation devices can be used for the second thickener 34. Theselection may depend on the suspended solids content in the digester 20and on the undigested matter particle size.

For digestate 24 with a solids content of up to 2 or 2.5% TSS, there arevarious solids separation options. Firstly, tubular cross flow membranescan be used without the need for polymer. Attempting to filter solidscontent significantly more than 2.5% with tubular membranes wouldgreatly reduce the membrane flux and an extremely large membrane surfacearea would be required. Typically, the solids content of the retainedsolids is doubled in the concentration. The membrane therefore becomesin practice a thickener. The digestate 24 is pumped through themembranes to create trans-membrane pressure and sufficient velocity tokeep a solids layer on the membrane surface from becoming thick. Thethickened sludge, for example at a 4 to 5% TSS concentration, returns tothe digester 20. Suspended solids capture is usually essentially 100%.

Secondly, flotation devices such as dissolved air flotation orcavitation air flotation may be used. In this case, polymer injection isused to create a floc that floats with the introduction ofmicro-bubbles. The floated sludge content is usually 4 to 5% TSS. Thefloat is pumped back to the digester 120. Suspended solids capture isusually about 98%.

Thirdly, drum thickeners may be used, also using polymer to create afloc. The floc is retained in a drum screen and the retained solids aremoved forward by a screw toward an outlet as they continue to thicken.Thickened solids concentrations of up to 8% are possible. Solids dropinto a hopper fed pump that returns them to the digester 120. Suspendedsolids capture is usually over 95%.

In the three configurations above, the liquid effluent 36 from thethickening device may be an essentially final plant effluent. However,the effluent 36 may undergo further treatment if necessary for dischargeto a receiving stream or sewer or for reuse. The liquid effluent mayalso be returned to the front end of a co-located wastewater treatmentplant in the case of a municipal sludge digester. In many cases, theliquid effluent 36 can also be treated to recover ammonia from it by achemical or biological process. Since tubular membranes removeessentially all of the suspended solids, they may be used with adownstream ammonia removal process that is not tolerant of suspendedsolids.

When the TSS in the digestate 24 or separator effluent 30 is higher than2.5%, a drum thickener is the preferred fine solids separation device.High consistency drum thickeners with an internal auger can reach 8 to12% TSS in the outlet. This allows sludge 38 to be thickened to twicethe entering concentration. The sludge 38 is then pumped back to thedigester 20 using a hopper fed positive displacement pump.

Optionally, a flotation device or drum filter may be followed with amembrane filter. The retentate of the downstream membrane filter isrecycled to the digester 20 and the membrane permeate is combined witheffluent 36 from the second thickener 34.

The digester 20 is preferably heated to maintain the temperature in amesophilic or thermophilic range. Heating is done using a recirculatingsludge closed loop 42 from the digester 20 into a heat exchanger 40, forexample a tube-in-tube or double spiral heat exchanger, and back to thedigester 20. The recirculating loop 42 uses a positive displacement pumpoperated with continuous or intermittent pumping. Temperature control isdone on the hot water side of the heat exchanger 40, automaticallyintroducing new hot water by means of a temperature control valve.

In FIGS. 2 and 3, anaerobic digestion systems 50, 52 are retrofit to awaste sludge digester located in a municipal wastewater treatment plant(WWTP). The systems 50, 52 increase the capacity of the existingdigester by operating at a higher solids content than conventional highrate municipal sludge digesters. In most cases, a WWTP has one or moreexisting digesters that could be converted to higher solids operationwith the addition of equipment and process modifications to be describedbelow, without building new tanks or adding volume to existing digestertanks. Alternatively, the systems 50, 52 may be constructed new for usewith a WWTP or as free-standing systems to treat sludges or other wastestreams.

FIGS. 2 and 3 show schematic process diagrams of systems 50, 52 havingdigesters with pre-thickening and recuperative thickening, respectively.The systems 50, 52 have been retrofit into the sludge digestion part ofa municipal sewage wastewater treatment plant (WWTP). The main processtrain, comprising for example a primary clarifier, process tanks and asecondary clarifier, are generally unchanged and not shown in FIGS. 2and 3.

Referring to FIG. 2, primary sludge A and waste activated sludge (WAS) Bfrom the WWTP are pumped to a sludge holding tank 60. The sludges A, Bare mixed by holding tank mixer 62 such as a mechanical or pumped jetmixer. Flow of the sludges A, B is also equalized in the holding tank60. Combined sludge C is taken from the holding tank 60 by a positivedisplacement sludge feed pump 64. A dilute polymer D is added to thecombined sludge C before it passes through a shear valve or static mixer66. The sludge and polymer blend E then passes through a pipe length 68to allow for flocculation.

The combined sludge C with flocculated solids is thickened in a rotarydrum or screw thickener 58. Filtrate H removed from the solids isreturned to the head works of the WWTP. Thickened sludge F is feed to atransfer pump 80, such as a hopper fed positive displacement pump, to bepumped to an existing anaerobic sludge digester tank 82. Additionalexternal, typically high strength, waste P may also be added to thedigester 82 for co-digestion from a source other than the WWTP.

Sludge in the digester tank 82 is converted by the biomass in thedigester into digestate. The contents of the digester tank 82 are mixedby one or more mixers 70 such as submersible hydraulic mixers. Themixers are accessed through service boxes 72 passing through a lid ofthe digester tank 82. Digester gas O is withdrawn through the serviceboxes 72 or the top of the digester tank 82 for treatment andutilization. An emergency overflow N is available to waste digestate toa drain if the digestate level becomes too high.

Digestate to be heated I is pumped through a heating loop with a sludgepump 78, typically a positive displacement pump. The digestate to beheated I passes through a sludge-water heat exchanger 56 to be heated bya flow of heated water. Hot water fed K flows to and through the heatexchanger 56. Hot water return L leaves the heat exchanger at a lowertemperate and is sent to a heater for re-heating and re-use. Warmdigestate J returns to the digester tank 82. Waste digestate M iswithdrawn from the digester tank 82 by a transfer pump 76 and sent forde-watering in de-watering equipment already existing in the WWTP.

System 52 in FIG. 3 uses similar components and streams as system 50 inFIG. 2 as indicated by the use of the same reference numbers andletters. However, in system 52 the primary sludge A, WAS B and anyexternal waste P for co-digestion flow into the digester tank 82directly. Digestate for recuperative thickening Q is taken from thedigester tank 82 and sent to the thickener 58 which is located in a sidestream loop that returns to the digester tank 82. Filtrate H is returnedto the headworks of the WWTP but may be treated on the way, for exampleto remove ammonia or phosphorous.

Retrofitting an existing WWTP digester to operate at a higher solidsconcentration enables a higher volatile solids reduction (VSR) andincreased digester gas O generation by extending the solids retentiontime in the digester tank 82. Additionally, the retrofit allows the WWTPto co-digest external waste P with larger proportions of high strengthstreams such as fats, oil and grease (FOG), food processing highstrength waste streams, or prepared slurries using the organic fractionof municipal, restaurant or grocery waste, or other mixtures.

Conventional high rate municipal WWTP digesters are fed a combination ofprimary sludge A, usually at 3% solids, and thickened waste activatedsludge B at 5% solids. The combination typically has 4% total solids ofwhich 80% is volatile. These existing digesters are typically designedfor 20 to 25 days of hydraulic retention time (HRT) and usually achieve50% to 55% VSR. The solids content of the digestate, which is thecontent of the digester tank 82, is usually about 2.5% solids. Digestatedynamic viscosity is usually 0.4 to 0.5 Pascal second (Pa·s) or 400 to500 centipoise (cP) at a shear rate of 20 to 30 over second (1/s) and 38degrees C. The volatile solids loading rate or organic loading rate(OLR) feeding 4% solids and allowing 20 days HRT is 0.09 lb VS/ft3-day(1.4 kg VS/m3-d).

If a digester with 20 days HRT fed with waste sludge A, B from a WWTP isused for co-digestion, usually no more than an additional 20% of thefeed flow, having less than 10% VS content, can be added as grease orother external waste P. Otherwise, the HRT is reduced excessively andthe slow growing methanogens exit the digester tank 82 at a rate higherthan that at which they reproduce, and this results in digester organicoverload and failure. SRT in a conventional CSTR digester withoutthickening is reduced with the HRT and soon passes below a minimumrequired for effective digestion of difficult wastes such as grease.However, with thickening a large inventory of anaerobic biomass ismaintained in the digester tank 82 and the hydraulic retention time isat least not reduced, and preferably increased. A co-digester withthickening can be operated at OLRs upwards of 0.27 lb VS/ft3-d (4.2 kgVS/m3-d), or three times the OLR of conventional high rate municipaldigesters. This also implies that three times the digester gas O will begenerated using the same digester tank 82 volume. If no co-digestion isdone but the digester is retrofit to operate at a high solidsconcentration, for example with the solids retention time doubled to 40days, a VSR of 65% is usually achieved. The combination of increasedfeed capacity (because reducing HRT does not necessarily reduce SRT) andincreased SRT allows a larger amount of co-digestion waste to be fed toa digester with thickening. For example, 30% or more of the VS loadingto the digester may come from co-digestion waste, without reducing theamount of waste sludge A, B fed to the digester tank 82 from the WWTP.

Digester operation with a higher solids content can be achieved in twoways. In FIG. 2, the combined primary sludge A and waste activatedsludge (WAS) B is pre-thickened to a solids content of 8 to 10% byremoving water from the combined sludge C prior to feeding the digester.In FIG. 3, recuperative thickening is used, which consists of thickeningthe digestate Q to remove water from it, and returning the biomass at ahigher solids content with thickened digestate R to the digester tank82.

Sludge pre-thickening can be done using drum thickeners or centrifugethickeners with the addition of polymer or flocculants. Sludgepre-thickening results in lower heat demand in the digester butincreases the ammonia concentration in the digester tank 82, since theammonia content is a function of the feed solids concentration. In manycases pre-thickening is more costly to implement in a WWTP retrofit thanrecuperative thickening. This is because primary sludge A and WAS B areusually pumped intermittently to several digesters, and so the holdingtank 60 is necessary to equalize the flows and feed both streams to acommon thickener. The viscosity of undigested sludge A, B or C is higherthan that of digestate for the same solids content. Therefore ifundigested combined sludge C is thickened to 8 or 10% solids, the highviscosity of the thickened sludge F will be make pumping and sludgeconveyance to the digester tank 82 difficult using existing sludgepiping. With pre-thickening, the hydraulic retention is also notdecoupled from the solids retention time. The HRT and SRT are the same,although the decreased feed flow rate extends both the HRT and the SRTfor a given digester tank 82 volume. However, when digesters are used toco-digest WWTP sludge A, B with external waste streams P, the externalwaste P adds water to the digester tank 82 and reduces both HRT and SRT.

Recuperative thickening effectively decouples SRT from HRT by retainingusually more than 95% of the suspended solids that exit with thedigestate sent for recuperative thickening Q, and returning thesuspended solids at about twice the solids concentration with thickeneddigestate R. This can be done by installing an individual recuperativedrum or centrifuge thickener 58 for each digester, or a common thickener58 for a group of digesters, and using polymer. Recuperative thickeningresults in higher digester heat demand as the water that is extracted isheated in the digester tank 82, although some heat recovery from thefiltrate H may be possible. Recuperative thickening also decreasesdigester alkalinity. However, recuperative thickening reduces ammoniaconcentration in the digestate relative to pre-thickening, which isbeneficial since excessive ammonia can inhibit digestion.

A digester operating with 5 or 6% solids (DS) will require recuperativethickening of its effluent to 10 or 12% solids. The viscosity of thethickened digestate R returning to the digester tank 82 is considerablylower than that of undigested sludge A, B, C at the same solids content.This results in easier pumping using existing pipes. Recuperativethickening is preferred for WWTP retrofits.

In both thickening scenarios, positive displacement pumps 64 are used tohandle thickened digestate R or thickened sludge F, and to circulate thedigestate I for heating through the heat exchanger 56, due to theviscosity increase (relative to a conventional WWTP digester) resultingfrom having sludge and digestate at a higher solids concentration.Digester mixing in the digester tank 82 is also affected by the increasein solids content and viscosity in the digestate. An increase in solidscontent from 2.5% to 5% will usually result in a ten-fold increase inviscosity. Digesters operated at 5 to 6% solids content have viscositiesof 5 to 7 Pa·s (5,000 to 7,000 cP). Digestate with this viscosity cannotbe properly mixed with pumping and jets or gas injection. Due to thehigh viscosity of the digestate and the operating temperature (38 deg C.if mesophilic or 55 deg C. if thermophilic), conventional electricsubmersible mixers are not adequate, as the electric motor tends tooverheat. Installing electrical equipment inside a digester tank 82 mayalso create risks of explosion associated with biogas in the headspace.High torque, low speed submersible mixers 70 are preferred. UTS ProductsGmbH in Lippetal, Germany manufactures high solids content submersiblemixers 70 driven by a hydraulic motor, also available through therelated company UTS Biogas Ltd., Cambridgeshire, UK. These mixers 70 arecontroller through the service boxes 72, which have a retractable skirtdesigned to isolate the service box 72 from the digester tank 82headspace. This allows safe mixer servicing without the need to emptythe digester tank 82 or stop operation. The service boxes 72 areinstalled on the digester cover, one on top of each mixer column guide,to access the mixers 70 for service and to enable repositioning orremoval of the mixers 70 without digester gas O in the headspaceescaping. Fixed digester covers are preferred for positioning theservice boxes 72. If a digester tank 82 has floating covers, these canbe converted to fixed covers, or replaced with double membrane covers,which are also suitable for installing service boxes 72 and submersiblemixers 70.

Each mixer 70 uses a 22 kW external hydraulic power unit and circulatesbiodegradable hydraulic oil, such that if leaks occur inside thedigester then the bacteria can degrade the non-toxic leaked oil. Usuallytwo or more mixers 70 are needed per digester tank 82, depending on thedigester tank 82 dimensions. The mixers 70 are located near the tankperimeter and directed to create a rotational movement of the digestateand also to reintroduce floating layers or crusts back into the bulk ofthe digestate. The UTS hydraulic mixers 70 and service boxes 72,although marketed primarily for agricultural and industrial digesters,are ideally suited for this type of retrofit. Alternatively, UTSelectrical mixers may be used. These mixers are described in Germanpatent application 10 2014 116 239.0 entitled “Verfahren zum Betreibeneiner RUhreinrichtung and eines Fermenters” filed on Nov. 7, 2014 whichis incorporated herein by reference.

Mixing is done intermittently, usually 20% of the time. Typical mixingintervals are 10 minutes ON and 40 minutes OFF, although other cyclescan be used. When co-digesting high solids food waste with municipalsludge, continuous mixing is counterproductive at high loading rates.Continuous mixing is not only unnecessary and more energy consuming, butactually reduces digester performance. Propionate, a volatile fattyacid, tends to accumulate in highly loaded digesters that arecontinuously and vigorously mixed, but is rapidly consumed under lowerintensity intermittent mixing conditions. Propionate inhibition withconstant mixing occurs in both mesophilic and thermophilic digesters.

The UTS hydraulic power units have the ability to drive up to 5 mixers70. The mixers 70 have automatic rotation reversal if a sudden torqueincrease is detected, which could be attributed to rags or anaccumulation of hair or other fibers in the mixer blades. This is knownas ragging. The vertical supporting columns of the mixers 70 allowflexibility in directing and positioning the mixers 70 so that themixing energy can be effectively used.

The preferred recuperative thickening equipment is an enclosed rotaryscrew thickener 58 with an internal screw, which is designed to thickensludge with a high initial solids content (3 to 6%). This is in contrastwith rotary screen thickeners that do not have an internal screw and aretypically used to thicken WAS (initially at about 1% solids). Screwthickeners are available from a few manufacturers. For pre-thickening,conventional rotary drum screens can be used, as offered by many vendorsand typically designed to thicken WAS. These units do not have aninternal rotating screw. A screen drum rotates with internal weldedflights moving the sludge forward as it drains. This type of thickenercan be used for combined sludge C pre-thickening prior to feeding thedigester, and some commercially available models can take the solidscontent to 8 or 10% starting at under 2% DS in the combined sludge C.Although rotary drum thickeners 58 may be used for recuperativethickening also, they are less efficient than rotary screw thickeners58. Enclosed thickeners 58 are preferred for odor control, as ammoniaand hydrogen sulfide would otherwise escape to the atmosphere.Thickening centrifuges can also be used, but these are more costly andrequire more energy to operate. Another suitable thickener forrecuperative thickening is a screw thickener as shown in US publication2014/0034574, which is incorporated by reference.

In recuperative thickening, a positive displacement pump 64 pumps fromthe digester tank 82 to the thickener 58. An in-line grinder can beinstalled in the pipe feeding the thickener 58 in cases where municipalsludge is co-digested with waste streams that contain fibers or largepieces. Dilute polymer D is injected upstream of the thickener 58 and ahigh shear static mixer or mixing valve 66 is used to disperse thepolymer D into the digestate Q. Flocculation is done in the pipe 68 asthe digestate Q and polymer D approach the thickener 58. Typical polymerdoses range from 4 to 6 kg per ton of solids. The thickener 58 removeswater and usually achieves 90 to 95% suspended solids recovery. Theeffluent filtrate H, with suspended solids in the 800 to 3,000 mg/Lrange, may be directed to the WWTP headworks for treatment in the mainliquid train.

When the combined sludge C is pre-thickened, the filtrate H does notcontain additional ammonia. In recuperative thickening, the filtrate Hcontains ammonia as a result of the organic nitrogen mineralized toammonia in the digestion process. With thickening, an increase innitrogen loading to the WWTP plant liquid train compared to conventionaldigestion is usually proportional to the increase in VSR, and may befurther increased by co-digestion. The higher the VSR, the more organicnitrogen is converted to soluble ammonia. In conventional sludgedigestion, and with both pre-thickening and recuperative thickening,ammonia returns to the WWTP in the filtrate or centrate from wastedigestate M dewatering. With pre-thickening, the amount of ammoniareturning with the filtrate or centrate from waste digestate Mdewatering is increased relative to a convention WWTP digester. Whenrecuperative thickening is used, a portion of the total ammonia returnedto the WWTP is carried with the thickener effluent H and the rest withthe waste digestate M dewatering filtrate or centrate. Wastewatertreatment plants that do not have the capacity to treat the additionalammonia in the filtrate H from recuperative thickening, or digestate Mde-watering effluent with pre-thickening, can use supplementarytreatments to remove or convert ammonia prior to returning the filtrateH or digestate M de-watering effluent to the main WWTP liquid train.

The thickened sludge F, R at 10 to 12% solids may drop from the thicker58 into a hopper-fed positive displacement pump 80 such as rotary lobeor progressive cavity pump. The displacement pump 80 sends solids withthe thickened sludge F, R to the digester tank 82. The digestion processresults in an accumulation of non digestible volatile solids, inertsolids, and biomass growing in the digester tank 82. These solids mustbe removed at a rate required to maintain the desired solids retentiontime. Waste digestate M is preferably taken directly from the digestertank 82 as there is normally existing piping, and it is easier to pump5% solids digestate than 10% thickened sludge F, R if it was to bewasted as from the outlet of the thickener 58. Waste digestate M goes todewatering with existing dewatering equipment in the WWTP.

Due to the increased viscosity in the digestate, modifications toexisting digester heating equipment in the WWTP are usually necessary.External tube-in-tube or double spiral heat exchangers 56 may be used.Tube-in-tube exchangers require large passages to reduce head losses andfacilitate cleaning, and also require internal static flow deflectors inthe sludge side to promote sludge turbulence and increase heat transferefficiency. This type of tube-in-tube exchanger is available from a fewmanufacturers and are preferred over spiral exchangers, which usuallyare more costly and have higher friction losses. Hot water K from aboiler or waste heat from a biogas engine generator may be used to heatthe sludge. Sludge is heated in a recirculation loop I, J moved by apositive displacement pump 78 with an in line grinder to reduce the riskof a heat exchanger 56 plugging with rags or fibers. The digestate isusually pumped continuously through the heat exchanger 56 and hot waterK, L is pumped through the exchanger 56 as needed to maintain thedesired temperature. This is done automatically with a temperaturecontrol valve.

The term polymer is used herein to refer to polymers useful sludge asthickening or dewatering aids or conditioners. A typical polymercomprises polyacrylamide. Typical polymer doses for thickening rangefrom about 5-6 kg/MT-TSS (metric ton of total suspended solids in thedigestate). Typical polymer doses for dewatering are about 10 kg/MT-TSS.However, the inventors have observed that polymer (i.e. cationicpolyacrylamides) recycled to a digester with solids during recuperativethickening retains at least some of its activity. Because of thisretained activity, the dosage in the thickener can be reduced a fewweeks after commissioning a digester with recuperative thickening from5-6 kg/MT-TSS to about 2-3 kg/MT-TSS. The dosage in typical dewateringequipment (i.e. centrifuges, belt filter presses and screw presses) canbe reduced to about 5-7 kg/MT-TSS.

FIG. 4 compares a standard digester without recuperative thickening anda digester with recuperative thickening (RT). By comparing the capillarysuction time (CST) it was found that up to 40% lower polymer doses couldbe applied when dewatering digestate produced with recuperativethickening (with polymer added to the recuperative thickener) ascompared to digestate from a conventional digester. To produce theresults shown in FIG. 4, daily grab samples were taken from thedigesters and coagulated and flocculated with a cationic polyacrylamide(CT). The flocculated sample was measured for CST. Typically, beltfilter presses and rotary presses require CST values <20 s to achievegood dewaterability without blinding the screen. The digester withrecuperative thickening was consistently able to achieve this low CSTeven when polymer dose was reduced from 10 kg polymer/MT-TSS to 6 kgpolymer/MT-TSS. The control conventional digester failed to achieve anappropriate CST value when the polymer dose was reduced. The twodigesters were fed the same mix of primary and waste activated sludge.The digester with recuperative thickening was operating at twice theorganic loading rate and producing twice the biogas as the conventionaldigester.

Waste sludge from a digester is typically dewatered before it is shippedfrom a plant to reduce its mass and volume. The dewatered digested maybe for example landfilled, applied to agricultural land as fertilizer orbiosolids, or composted. Accordingly, many existing digesters alreadyhave dewatering equipment. As an alternative to or in addition torecuperative thickening by way of returning thickened digestate to adigester, recuperative thickening can be achieved by returning dewatereddigestate to the digester. Dewatered digestate cake can be produced, forexample, in a centrifuge or belt press. The dewatered digestate containsviable bacteria and returning it the digester increases the HRT of thedigester. While dewatering is likely to be more energy intensive thanthickening for a given increase in HRT or solids content in thedigester, it may be economical to use existing dewatering equipment,possibly with modifications to reduce the solids content of the cake butallow for increased throughput. The dewatered digestate may have a veryhigh solids content, for example 20-35% TS, and typically cannot bepumped. However, the dewatered digestate can be conveyed, for example byauger, directly to the digester or blended with the feed liquid to makea pumpable mixture.

Dewatered feed sludge may also be used in place of thickened feedsludge. Wastewater treatment plants (WWTP) often have dewateringequipment used for dewatering waste activated and/or primary sludge.Dewatered sludge from the WWTP can be fed with ordinary sludge (blendedtogether or in two separate streams) to produce a combined feed having asolids content similar to pre-thickened WWTP sludge and fed to thedigester. For example, as described above, typical combined sludge froma WWTP may have about 2-4% TS. The combined sludge can be thickened to 8to 20% TS and fed to the digester. Alternatively, dewatered WWTP sludgecake at 20-35% TS (for example 25% TS) can be blended with the combinedsludge at 2-4% TS in proportions that produce a feed mixture at 8 to 20%TS.

In one process, primary, waste activated or combined sludge from a WWTPis pre-thickened and fed to an anaerobic digester without a co-digestedwaste stream (or with only a small, i.e. 10% of the feed VS or less,co-digested waste stream) and without recuperative thickening.Pre-thickening feed to the digester does not decouple HRT from SRT butincreases both HRT and SRT. The WWTP sludge may be thickened, or acombination of WWTP sludge and dewatered WWTP sludge (blended or fedseparately) may be used for the feed.

12% TS is usually the limit for feeding combined primary sludge andwaste activated sludge (WAS) to a digester alone. WAS typically containsorganic nitrogen in bacterial cells, for example at about 10% of thevolatile solids (VS) in the WAS. As the VS are degraded in the anaerobicdigestion process, the nitrogen mineralizes to ammonia. The ammoniaconcentration in the digestate when feeding 12% TS WWTP sludge may reach3000-3500 mg/L. Feeding a higher concentration of combined WWTP sludge(particularly WAS) can cause the ammonia concentration to rise to alevel where the digestion is inhibited by ammonia toxicity.

Feeding at least a portion of dewatered WWTP sludge, or WWTP sludgehighly thickened in for example a belt press or centrifuge, to thedigester allows a feed concentration of 12-20% TS or 15-20% TS. Thedigester may then operate at 7-10% TS. Although operating at such highsolids concentrations creates difficulties, there are some advantages.For example, relative to recuperative thickening, there is less loss ofheat from the digester. For another example, the high ammoniaconcentration in the digestate enables nitrogen recovery by way ofstripping ammonia gas from waste digestate.

One of the difficulties is in dewatering the digestate. It is verydifficult to mix dewatering aids such as polymers into high solidsdigestate because of its viscosity. The digestate is therefore dewateredin two stages. The first stage is performed without polymer, for examplein a screw thickener. In the second stage, the filtrate from the firststage is treated with polymer, for example in a centrifuge or screwthickener with smaller screen openings. Another difficulty is that theammonia concentration in the digester may reach as much as 5000-5500mg/L. Micronutrients, for example a source of volatile carbon toincrease the C:N ratio, are added to the digester to alleviate toxicityfrom the high ammonia concentration. Another difficulty is that thedigestate is hard to mix. However, electric mixers with a permanentmagnet synchronous motor as described in German patent application 102014 116 239.0 entitled “Verfahren zum Betreiben einer RUhreinrichtungand eines Fermenters” filed on Nov. 7, 2014, is incorporated herein byreference, have been found able to mix digestate at 8, 9 and even 10%TS.

As discussed above, when treating WWTP sludge, one process includes feedpre-thickening or dewatering but no recuperative thickening. WWTP is fedat 12-20% TS to a digester. The digestate has about 6-12% TS.Micronutrients are added to alleviate toxicity from the high ammoniaconcentration in the digester. Two-stage digestate dewatering is used toaccommodate the high solids content of the digestate. In the firststage, no polymer is added and a relatively coarse screen (or otherseparation device) is used. In the second stage (treating the liquidfraction produced by the first stage), polymer is used and relativelyfine screen (or other separation device) is used. Another processincludes recuperative thickening but not feed thickening. WWTP sludge isfed to a digester at 1-5% TS, for example 3-4% TS. The digestate hasabout 4-6% TS. Higher digestate TS is possible with two-stagedewatering, but in this process one stage dewatering with polymer addedis preferred since a separate recuperative thickener operating in onestage with polymer addition is also preferred. The amount of polymerused for recuperative thickening may be about 2-3 kg/MT-TSS. Some ofthis polymer is recycled to the digester and so lowers the polymer doesrequired in both the recuperative thickener and waste sludge dewatering.In both of these processes, high solids hydraulic or electric mixersenable mixing in the digester despite the high solids content. The feedto either process may be WWTP sludge only or WWTP sludge and an externalwaste stream.

U.S. Pat. No. 9,181,120 and US Publication 2012145627 are incorporatedby reference.

Unless stated otherwise or apparent form the context, solids contents orconcentrations mentioned above are dried solids (DS) measurements whichwould be the same as a total solids (TS) measurement. In digestate, theDS is roughly 10% higher than total suspended solids (TSS) and the totaldissolved solids (TDS) is typically 2500 to 4000 mg/L (0.25 to 0.4%).For example, a 5% DS digestate may have 46,000 mg/L of TSS and 4000 mg/LTDS. Accordingly, solids contents or concentrations, unless specifiedotherwise, can generally be interpreted as TSS without causing amaterial difference in the process.

The descriptions of processes and apparatus above are to provide atleast one example of an embodiment within each claim but not to limit ordefine any claim. However, multiple processes and apparatus have beendescribed above and it is possible that a particular process orapparatus described above is not within a specific claim. Processparameters are given as examples of how a plant may be operated and arenot meant to limit a claim unless explicitly recited in a claim. Otherprocesses for similar applications might operate at parameters withinranges that are 50% or 100% larger in both directions than the parameterranges described above, or within a 50% or 100% variation from a singleparameter described above. If one or more elements or steps describedabove are used to treat other wastes or under other conditions, then oneor more process ranges described above might not be suitable and wouldbe substituted with other appropriate parameters. Various sub sets ofthe unit processes described in relation to plant 100 can be used inother treatment plants. Various sub sets of unit processes in thetreatment plants described above may also be combined in ways other thanthose described to produce different treatment plants. The descriptionof one process or apparatus may be useful in understanding anotherprocess or apparatus. Words such as “may”, “preferable” or “typical”, orvariations of them in the description above, indicate that a processstep or apparatus element is possible, preferable or typical, accordingto the word used, but still optional and not necessarily part of anyclaimed invention unless explicitly included in a claim.

We claim:
 1. A process for treating WWTP sludge comprising the steps of,feeding the WWTP sludge at 12-20% TS to an anaerobic digester; operatingthe anaerobic digester at 6-12% TS; and, dewatering digestate in twostages.
 2. The process of claim 1 further comprising adding nutrients tothe digester in an amount effective to alleviate ammonia toxicity. 3.The process of claim 1 further comprising mixing the digester with ahydraulic or electric motor mixer.
 4. A process for treating WWTP sludgecomprising the steps of, feeding the WWTP sludge at 1-5% TS to ananaerobic digester; operating the anaerobic digester at 4-6% TS; and,providing recuperative thickening for the digester by way of a sidestream thickener; and, adding a polymeric thickening aid in the sidestream.
 5. The process of claim 4 wherein the polymeric thickening aidis added at 3 kg/MT-TSS or less.
 6. The process of claim 4 wherein wastedigestate is dewatered separately from the side stream.
 7. The processof claim 6 wherein a polymeric thickening aid is added at 7 kg/MT-TSS orless for dewatering.
 8. The process of any claim 4 further comprisingmixing the digester with a hydraulic or electric motor mixer.